When a seamless tube is produced by a Mannesmann-mandrel mill tube-making machine, in an elongation rolling process of mandrel mill rolling, the rolling is performed by grooved rolls in a multi-stand rolling mill which feeds a blank tube in an axial direction while constraining the outer surface of the blank tube, along with a mandrel bar which constrains the inner surface of the blank tube.
When performing the elongation rolling, generally, a lubricating film predominantly composed of a solid lubricant such as graphite etc. is formed on the surface of the mandrel bar in advance; however, since the mandrel bar and the inner surface of the blank tube come into a condition of severe sliding friction, it is not easy to realize a complete lubrication state, and surface damages such as repeated wear, seizing, surface roughening/deterioration, and cracks are liable to occur.
As the surface condition of the mandrel bar which is used in a repeatedly circulated manner degrades depending on the use thereof, the mandrel bar will be temporarily displaced from the circulation line so that surface conditioning is performed; in this connection, particularly in the rolling of seamless tubes made of high-alloy steel and stainless steel as material grade, frequencies of surface conditioning of the mandrel bar are increased, resulting in a factor to deteriorate productivity. For this reason, conventionally, countermeasures to extend the useful life of the mandrel bar and to reduce frequencies of surface conditioning of the mandrel bar have been performed.
For example, Patent Literature 1 proposes a mandrel bar for hot-rolling a seamless tube, in which a scale layer having a thickness of 6 to 20 μm is formed on the surface of the mandrel bar of which centerline average roughness Ra is not more than 20 μm. The formation of the scale layer is performed by keeping the mandrel bar at 600 to 650° C. under an oxidizing atmosphere for a predetermined time period.
Moreover, Patent Literature 2 proposes a mandrel bar for producing hot rolling seamless tubes having a nitrided layer, of which centerline average roughness in the axial direction is 0.5 to 5.0 μm, on the surface of the substrate material. It is stated that by increasing the surface strength by forming a nitrided layer on the surface, and further controlling an optimum surface roughness in the nitrided mandrel bar, it is made possible to achieve an excellent service life even when performing mandrel mill rolling of a seamless tube made of high-alloy steel as material grade, and to significantly improve the inner surface quality of a resultant product.
However, applying the processing to form the scale layer and the nitrided layer as described in Patent Literature 1 or 2 to a hot-rolling tool having a long and heavy body, such as a mandrel bar for producing hot rolling seamless tubes, requires dedicated facilities therefor and is not necessarily easy.
Patent Literature 3 proposes a mandrel bar for producing hot rolling seamless tubes, in which a Cr-plated film having a thickness of 60 to 200 μm is formed on the surface of the substrate metal. It is stated that by increasing the thickness of the plated film than before, the service life of the mandrel bar is significantly improved even when subjected to the rolling of high-alloy steel containing not less than 2 wt % of Cr; however, the formation of a Cr-plated film is required and therefore an increase of cost is inevitable.
Further, Patent Literature 4 discloses a method for improving the service life of mandrel bar, in which upon regenerating the mandrel bar which has undergone wear and deterioration of the surface thereof and has become non-usable, the mandrel bar after use is subjected to heat treatment at a temperature, as being not more than the initial tempering temperature during new production, to homogenize the surface hardness, and after not less than 0.06 mm of the surface of the mandrel bar is ground or machined and then polished, a scale film for seizure resistance is formed thereon.
According to the method of Patent Literature 4, it is possible to significantly reduce the amount of machining of outer diameter compared with a conventional method for regenerating a mandrel bar in which after lubricant is removed, the diameter is machined by about 5 to 20 mm, thereby significantly improving the service life and specific consumption of the mandrel bar. However, specific processes such as for applying pre-heat treatment on the mandrel bar become needed.
Further, Patent Literature 5 proposes a lubricant composition for high-temperature rolling, which is obtained by blending one or more particulate oxide-based laminar materials selected from potassium tetrasilicic mica, sodium tetrasilicic mica, and the like, and one or more binders selected from boron oxide, alkali metal borate, and the like at a predetermined weight ratio. It is stated that this lubricant composition exhibits excellent lubricity and, in addition, does not cause inconveniences such as forming a carburized layer and a phosphorized layer in the material to be processed since it does not include graphite and phosphoric acid.
By the way, conventionally, when producing a seamless steel tube made of high-alloy steel and stainless steel with a mandrel mill, a so-called launching rolling is performed wherein a newly made mandrel bar is used for the first time, graphite is applied on the surface of the mandrel bar as a lubricant and a seamless tube made of carbon steel as workpiece is rolled.
In other words, that is a method in which since seizing occurs if tubes made of high-alloy steel and the like are rolled by using a newly fabricated mandrel bar; in the first stage of the use of mandrel bar, the launching rolling for initial use is performed for a predetermined number of times for tubes made of carbon steel as workpieces, thereby forming a film made up of graphite and a scale, the film having a high adhesiveness, on the surface, causing consolidation/densification of the structure, reducing the friction coefficient of the surface of the mandrel bar, and thereafter rolling a steel tube made of high-alloy steel, stainless steel, or the like by using the mandrel bar. This will make it possible to produce a hot seamless steel tube made of high-alloy steel and stainless steel by using general graphite as a lubricant without spending surplus processes and man hours.
However, growing demands in recent years on high-alloy steel tubes, such as of 13-Cr steel, and stainless steel tubes make it difficult to obtain the workpiece (carbon steel) for the launching rolling for initial use, and it is necessary to reduce friction coefficient and improve seizure resistance for newly fabricated mandrel bars right in the first stage of use.